The objectives of this study are: (a) to measure the effects of pregnancy upon the maternal physiologic responses to standard exercise, with particular attention to maternal cardiovascular and respiratory changes; (b) to study the effects of maternal exercise upon uterine blood flow and fetal oxygenation; and (c) to evaluate the effects of limited maternal oxygen transport capacity (caused by heart disease or anemia), and of an increased uterine oxygen need (caused by twin or triplet pregnancy) upon the maternal and fetal responses to exercise. Measurements will be made in the same individuals at several stages of pregnancy; after pregnancy; and, whenever possible, before pregnancy. At each study, measurements will be made at rest, during exercise and during recovery. In humans, measurements will include rates of oxygen consumption and carbon dioxide production, respiratory frequency and tidal volume, end-expiratory PCO2, maternal heart rate and fetal heart rate. In goats, in addition to the measurements listed for humans, the distribution of cardiac output at rest and during standard exercise will be measured near term by the injection of isotope-labeled microspheres: if fetal tachycardia accompanies or follows exercise, fetal blood samples will be drawn through indwelling catheters to evaluate fetal oxygenation and acid-base balance (in subsequent studies). Findings will be correlated with the stage of gestation, maternal age, maternal weight and height, the hemoglobin concentration in maternal blood, the size of the newborn and placenta, and the hemodynamic response to exercise of the mother three months after delivery. Correlations of the responses to exercise will also be made with the number of fetuses present.